The present invention relates to liquid crystal display device which functions both as a cholesteric reflective flat-panel display and a solar cell assembly to provide electrical energy to power the display and associated electronics and, more particularly, to a liquid crystal display device including a passive matrix, bistable, cholesteric liquid crystal display having one or more solar cells disposed in alignment with the display and adjacent a substrate bounding a layer of cholesteric liquid crystal material, the one or more solar cells providing electrical energy to power display electronics.
Typically, a reflective liquid crystal display comprises a single layer of liquid crystal material sandwiched between adjacent inner surfaces of generally planar substrates. In matrix type liquid crystal displays, on the inwardly facing surface of one of the substrates is disposed a set or array of parallel column electrode segments (column electrodes) and on an inwardly facing surface of the other of the substrates is a set or array of parallel row electrode segments (row electrodes), extending generally perpendicular to the column electrodes. The row and column electrode segments (also referred to as xe2x80x9crow and column electrodesxe2x80x9d) are spaced apart by the thin layer of liquid crystal material. Display picture elements or pixels are defined by regions of liquid crystal material adjacent the intersection of the row and column electrode segments.
Upon application of a suitable electric field, a pixel of a display will assume either a reflective or a non-reflective state. A pixel, P(xi,yj), formed at the overlapping or intersection of the ith row electrode segment and the jth column electrode segment is subject to an electric field resulting from the potential difference between a voltage applied to the ith row electrode segment and a voltage applied to the jth column electrode segment.
Recent advances in liquid crystal material research have resulted in the discovery of bistable cholesteric (also referred to as chiral nematic) liquid crystal display devices. Cholesteric liquid crystal display materials are able to maintain a given reflective state (reflective or nonreflective) without the need for the constant application of an electric field. In a reflective cholesteric liquid crystal display, the reflectivity of an image pixel depends upon the configuration or texture of the liquid crystal material (e.g., planar, focal conic, homeotropic configurations) defining the image pixel. Moreover, the state of the liquid crystal material may be changed upon imposing an appropriate electric field across the liquid crystal material for an appropriate period of time. This is accomplished by appropriately energizing the row and column electrodes defining an image pixel so as to generate an electric field having a desired magnitude (that is, a desired root mean square (rms) voltage) for a desired period of time. If the panel or substrate furthest from the viewer is painted with a black material, a pixel with a low reflectance or nonreflective state will appear as a black area to the viewer. A pixel in a high reflectance state will appear to the viewer as a visible colored area in the display.
Display driver circuitry is coupled to the vertical and horizontal electrodes. Operating under the control of a logic and control unit, the display driver circuitry energizes the row and column electrodes with appropriate voltage waveforms such that an appropriate voltage across each pixel is generated. The voltage across a pixel will either cause it to remain in its present state of reflectance or change its state of reflectance. The image generated by the display pixels may be modified by changing the state of selected pixels. In this way, text or image data can be presented for viewing on the display.
Certain prior art calculators and watches have included both a liquid crystal display and a solar cell assembly to provide electrical energy to the device electronics. However, in such devices, the display and the solar cell or cells have been disposed in different areas of the device, that is, the display area and the solar cell area do not overlap. This requires a device with a surface area large enough to accommodate both the solar cell area and the area of the display. This type of configuration is disadvantageous in small sized hand held devices were surface area is at a premium.
The present invention is directed to a cholesteric liquid crystal display utilizing a solar cell assembly as a power source for powering the display electronics. The display includes a front substrate, closest to a viewer, a back substrate and a thin layer of cholesteric liquid crystal material sandwiched therebetween. On an inner surface of the front substrate (that is, the surface of the substrate adjacent the liquid crystal material) is disposed a set or array of electrode segments and on an inner surface of the back substrate is a set or array of electrode segments, extending generally perpendicular to the column electrode array. In one configuration of the present invention, the display is a matrix display and one set of electrode segments comprises a set of parallel row electrode segments and the other set of electrode segments comprises a set of parallel column electrode segments. The row and the column electrode segments are substantially orthogonal. This arrangement of perpendicular row and column electrodes results in an orthogonal pattern of image pixels (orthogonal display). It should also be appreciated that the present invention is equally suited to providing power to other types of liquid crystal displays in addition to orthogonal displays such as, for example, segmented displays. While a segmented display also includes electrodes which may be fabricated in various shapes and disposed in orthogonal or non-orthogonal orientations to generate desired image configurations. For example, in a segmented liquid crystal display electrode segments may advantageously be disposed to create a seven segment numerical display. In other segmented displays, electrode segments of irregular shape may be used to generate an icon on the display. Moreover, it should additionally be appreciated that the concept of the present invention of using a solar cell assembly in a cholesteric liquid crystal display to provide power to display electronics is equally applicable to active matrix cholesteric liquid crystal displays in addition to passive matrix cholesteric liquid crystal displays. An actively driven matrix cholesteric liquid crystal display is one in which where each of the image pixels is driven individually by an active circuit component, e.g., a transistor. An active matrix Ch-LCD is disclosed in an article entitled xe2x80x9cAmorphous Silicon Thin-Film Transistor Active-Matrix Reflective Cholesteric Liquid Crystal Display,xe2x80x9d authored by J. Y. Nahm, T. Goda, B. H. Min, T. K. Chou, J. Kanicki, X. Y. Huang, N. Miller, V. Sergan, P. Bos and J. W. Doane and published in the Proceedings of the 18th International Display Research Conference, Seoul, Korea, September 1998, pages 979-982. The aforesaid active matrix Ch-LCD article is incorporated herein in its entirety by reference.
In one embodiment of an orthogonal Ch-LCD, display drive electronics include a set of row driver electronics electrically coupled to the row electrode segments that controls energization of all of the row electrodes and a set of column driver electronics electrically coupled to the column electrode segments that controls energization of all of the column electrode segments in the plurality of sets of column electrodes. The sets of row and column driver electronics constitute a single set of drive electronics.
The cholesteric or chiral nematic liquid crystal material is unique and advantageous in that it permits illumination incident on the display to pass through the liquid crystal material and impinge upon the solar cell. Typical nematic liquid crystal material displays such as twisted nematic (TN) or supertwisted nematic (STN) are dissimilar in that they function as a light shutter wherein light that passes through the nematic material is reflected toward the viewer by a reflector in back of the layer of nematic material. However, cholesteric liquid crystal material permits a percentage of incident radiation to pass through the liquid crystal material whether the material is in its highly reflective state (corresponding to the twisted planar configuration of the material) or in its low reflectance state (corresponding to focal conic configuration), or any state therebetween. Radiation that passes through pixels of the display in the focal conic state is absorbed by a black layer at the back of the display for providing contrast with light reflected from pixels in the reflective twisted planar state.
The cholesteric liquid crystal display (Ch-LCD) technology, be it an active matrix, passive matrix or segmented display, is ideal for the incorporation of a solar cell assembly in that most of the light incident on the display is available for generating electrical energy. This is not true for other reflective liquid crystal display technologies where only a few percent, if any, is available. In a Ch-LCD, the incident light is reflected by the liquid crystal material itself. In the reflective state, incident light is decomposed into its right and left circular components with only one of the components or 50% of the light being reflected. The other circular component of incident light passes through the liquid crystal material. A monochrome Ch-LCD therefore only reflects 50% of one color with a bandwidth of about 100 nanometers (nm.), the rest of the visible light, i.e., the other colors and the other non-reflected circular component are available for conversion to electrical power by the solar cell assembly. All of the incident light that passes though the cholesteric liquid crystal material can be equal to about 75% of incident light (or incident light intensity) for a monochrome Ch-LCD. Even in a full color Ch-LCD, which is comprised of stacked liquid crystal cells, those colors that are not being reflected in a particular image pixel are available for conversion to electrical power by the solar cell assembly and in such Ch-LCDs, about 65% to 75% of the incident light can be available for conversion to electrical energy by the solar cell assembly, depending on the image.
The cholesteric Ch-LCD display of the present invention advantageously utilizes the solar cell assembly in contrast to twisted nematic (IN), supertwisted nematic (SIN), ferroelectric (FLC) and other liquid crystal displays which use polarizers to generate an image on the display. In such displays that make use of polarizers, 50% of the incident light is absorbed by a polarizer and not available for conversion to electrical power. Also, such displays use a mirror on the backplane or back substrate to reflect the incident light (as opposed to the cholesteric liquid crystal material reflecting a specific color) and that reflected light is similarly not available for conversion to electrical power. Likewise, guest host type liquid crystal displays absorb light not being reflected and are, therefore, unsuitable for advantageously utilizing a solar cell assembly to supply power to the display electronics.
Cholesteric liquid crystal displays also have another advantage over other display technologies in that they possess bistable memory and do not require any electric power at all to maintain an image on the display, electric power is only required to change the image, i.e., change the reflective state of selected image pixels. Liquid crystal displays utilizing TN and STN technologies need to be refreshed about 60 times per second to maintain an image on the display. In devices that are not required to show moving video images, the power consumption is substantially less in Ch-LCDs than other liquid crystal technologies. Thus, electrical energy provided by a solar cell assembly theoretically does not have to be xe2x80x9cusedxe2x80x9d to refresh the display image if the image does not change, instead, such electrical energy is available for powering other electronics of the display.
The foregoing features make Ch-LCD uniquely advantageous for utilizing a solar cell assembly. The combination of LCD and solar cell assembly is especially attractive for hand held and other portable devices wherein the combination of Ch-LCD and solar cell assembly will facilitate size reduction of the device.
In a first preferred embodiment of the present invention, a light absorbing solar panel assembly comprising one or more solar cells is affixed to the outer surface of a transparent back substrate (that is, the surface of the back substrate away from the liquid crystal material). This approach is useful for solar cells whose front surface is an appropriate black or dark color as to provide contrast for pixels in the focal conic configuration (low reflectance state). Preferably, a thin layer of index matching optical material is applied between the solar panel assembly and the back substrate to provide suitable optical coupling and to reduce reflections at the substrate/solar cell interface.
In a second preferred embodiment of the present invention, a visibly blackened or colored but infrared (IR) transmissive layer or coating comprising IR transmissive ink is applied to the outer surface of the back substrate and a light absorbing solar cell is affixed to the outer surface of the back substrate, that is, the IR transmissive layer is sandwiched between the outer surface of the back substrate and the solar cell. The IR transmissive layer is provided upstream of the solar cell to ensure proper contrast in the event that a solar cell having an undesirable reflectivity is used. The IR transmissive layer provides a dark background for pixels in the focal conic state and absorbs visible light (in the range of 0.38 to 0.78 micrometers (xcexcm.)) but allows a majority of radiation having wavelengths in the near infrared range (typically 0.75 xcexcm. to 1.5 xcexcm.) as well as radiation of the middle and far infrared ranges (typically 1.5 xcexcm. to 1000 xcexcm.) and greater to be absorbed by the solar cell assembly.
In a third preferred embodiment of the present invention, the solar cell assembly of the display includes a solar panel assembly which functions as the back substrate or panel of the display. Advantageously, the solar panel assembly includes one or more solar cells comprising a plastic or glass base material with a solar radiation absorbing material coated or bonded on an outwardly facing surface of the base material. The opposite surface of the base material will include horizontally spaced apart ITO electrode segments affixed thereto such that the solar cell and the electrode segments share an opposite side of a common base material.
In a fourth preferred embodiment of the present invention, the display is a stacked reflective cholesteric liquid crystal display, for example a triple stacked display providing a color display an RGB (red, green and blue) color display with a solar cell assembly functioning as a back substrate or panel of the display. Alternately, the display could be a double stacked display offering the advantage of improved brightness for the pixels in the xe2x80x9conxe2x80x9d or reflective state or for providing night vision capabilities.
In one aspect of the present invention, a liquid crystal display is disclosed comprising: a) a layer of chiral nematic liquid crystal material; b) first substrate and a spaced apart second substrate, an inner surface of the first substrate and an inner surface of the second substrate bounding said liquid crystal material layer, the first substrate being closer than the second substrate to a viewer of the display; c) a first set of conductive electrodes disposed on the inner surface of the first substrate and a second set of conductive electrodes spaced apart from the first set of electrodes and disposed on the inner surface of the second substrate bounding said liquid crystal material layer; d) display driver circuitry electrically coupled to the first and the second set of conductive electrodes for generating desired voltage differentials between electrodes of the first set of conductive electrodes and electrodes of the second set of conductive electrodes; and e) a solar cell assembly including one or more solar cells electrically coupled in series and positioned adjacent said second substrate and electrically coupled to the display driver circuitry, the solar cell or cells receiving illumination passing through the first substrate and said liquid crystal material and converting the illumination incident on the solar cell or cells to electrical energy to supply power to the display driver circuitry.
The liquid crystal display includes an energy storage device coupled to and providing power to the display driver circuitry. The solar cell assembly is electrically coupled to and supplies power to the energy storage device. Preferably, the solar panel assembly, comprising one or more solar cells, is disposed adjacent an outer surface of the second substrate or, alternatively, comprises the second substrate, i.e., the solar cell is comprised of a plastic or glass material that functions as the second substrate.
In another aspect of the present invention, a liquid crystal display device is disclosed comprising chiral nematic liquid crystal material, cell wall structure communicating with liquid crystal material to form focal conic and reflective twisted planar textures that are stable in the absence of an electric field, a solar cell device for converting electromagnetic radiation that has passed through liquid crystal material into electrical energy, and means for applying an electrical field to liquid crystal material to place at least a portion into at least one of the focal conic and twisted planar textures.